Is or was there life on Mars? To clarify this question, one goal of Mars missions is to detect biomolecules on the red planet. But can these actually be identified with the methods currently used? A new study now shows that the harsh conditions in space are probably changing the molecules on the surface of Mars in such a way that they can no longer be detected by the Mars rovers' measuring devices. In contrast, detectable biomolecules could well have been preserved in deeper soil layers, according to the study.
In February 2021, the Mars rover "Perseverance" landed on our neighboring planet. One of his goals: The search for fossil or current evidence of life. Previous missions had already shown that there was water on Mars three to four billion years ago, as well as the elements carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus - important prerequisites for life. The current mission is now trying to find biomolecules in the sediments. These would be clear indications that there were, in fact, living beings on Mars. To do this, Perseverance uses a method called Raman spectroscopy. Samples are irradiated with laser light. The scattering of the light provides information about the composition of the material. So far, however, no biosignatures have been discovered.
In search of traces of life
But would it be possible in principle to use this method to detect traces of past or current life on Mars? Or do biomolecules change so much under space conditions that, unlike on Earth, they can no longer be detected with Raman spectroscopy? To find out, a team led by Mickael Baqué from the German Aerospace Center (DLR) in Berlin subjected seven molecules to a long-term stress test in space. "We selected the molecules based on how relevant they are likely to be as biosignatures on Mars and based on how well they can be detected using Raman spectroscopy based on current knowledge," the researchers explain.
The choice fell on chlorophyllin, a derivative of the green leaf pigment chlorophyll required for photosynthesis; the vitamin A precursor beta-carotene; the pigment melanin, which is found in our skin, among other things; the insect shell building block chitin; the plant support molecule cellulose and the two antioxidant plant pigments naringenin and quercetin. The researchers exposed these molecules to space conditions on the outside of the International Space Station (ISS) for 469 days. The intensive ionizing and UV radiation, the day-night cycle that changes every 90 minutes and the extreme temperature fluctuations associated with it had an effect on the samples.
Conditions simulated on Mars
To simulate the different soil layers of Mars, Baqué and his colleagues mixed the molecules with regolith, a material modeled after the Martian soil. The samples were packed in three layers under highly transparent glass in such a way that only the top layer was directly exposed to space conditions, while the two layers below were at least somewhat shielded from the strong radiation - similar to deeper layers of soil on Mars. After the experiment, the samples were shielded from other light and environmental influences, returned to Earth and examined there using Raman spectroscopy.
"Our results are the first systematically measured Raman signatures, quasi fingerprints of isolated biomolecules exposed to space in low Earth orbit," explains Baqué. For all samples that were located directly on the surface, it was shown that the UV radiation had changed the biomolecules so much that their signals could hardly be identified with Raman spectroscopy. The spectra of the deeper layers, on the other hand, had changed only slightly.
Importance for current and future missions
"This confirms that we can use Raman spectroscopy as a fast and non-destructive measurement technique to search for traces of life on Mars - especially in the underground, which is shielded from UV radiation," says Baqué. His colleague Jean-Pierre Paul de Vera adds: “This finding is of fundamental importance for those Mars missions looking for biosignatures under the Martian surface. However, biosignatures directly on the surface are more difficult to identify for Raman spectroscopy. But there are other methods that are even more suitable today.”
The results will benefit both the current Mars mission and the future planned Rosalind Franklin mission, which will also look for signs of life on Mars.
Source: Mickael Baqué (Institute for Planetary Research, DLR, Berlin) et al., Science Advances, doi: 10.1126/sciadv.abn7412